1. Make a nice view in advanced find with one or more columns to identify the records and the columns you want to enrich.
In this example I use the First and the Last Name to identify my Contacts and I want to enrich the Home Phone.

2. Export this view to Microsoft Office Excel and add a column to the left with the name of the entity in the heading. Put the Guids corresponding to the records in this column. And enrich your data.

3. Save your Excel file as CSV (comma delimited) and Create a new Data Import. Select your saved CSV, select Data Delimiter ‘none’ and Field Delimiter ‘Comma (,)’. On the next screen select your entity (in this case Contact).
As you will see at the top of the screen the possibility to ‘Enrich data by updating records rather than creating new records.’ is given.
This was the big surprise:

And the full screen:

4. After clicking a couple of times on the next button and some patience you can open your Advanced Find view to evaluate your results.

5. Next to that, if you open the Import record, you can see that the Contact records are actually updated and not created.

A small addition to this post. I got a lot of questions on how to get the GUIDS. If you want to get the GUID you can make an export directly from the database. This is not really userfriendly, but it works.
Furthermore my article was more to show that the option within the standard application still exists, than to really give my opinion on the best way how to enrich data.

Another (better) option to enrich data is to use a free tool from codeplex: http://mscrmimport.codeplex.com/

Have fun with it, but use it with care.